Abstract: The heterogeneity of particle and pore distribution and its resulting bottleneck effect are the main reasons for the hysteresis effect of soil and water characteristic curves (SWCC) on unsaturated soil. In this paper, the fractal theory is introduced to consider the microscopic fractal characteristics of the pore sizes and the seepage path of unsaturated soils. A capillary model is proposed for describing the seepage of water in unsaturated soil. In the proposed model, the pore size of unsaturated soil is simplified into a series of capillary tubes with different pore sizes, and the pore sizes and tortuosity are assumed to conform to fractal laws. On this basis, the equation of saturation Se-head height h of unsaturated soil during the wetting and drying processes is obtained to describe the characteristic equation of the hysteresis effect of SWCC. In addition, the characteristic equation of saturation Se-relative hydraulic conductivity coefficient Kr is established. Compared with the existing methods, this model can better simulate the hysteresis effect of SWCC of unsaturated soil. Finally, the essence of the hysteresis effect of unsaturated soil in the wetting and drying processes is compared and analyzed. It is revealed that the fundamental reason for the hysteresis effect is the heterogeneity of the pore size in the soil.

Key words: unsaturated soil, hysteresis effect, mesoscopic capillary model, fractal theory